What's in the muesli mix? I don't think these are normally safe/healthy for guinea pigs.
Generally if our piggies wheek a lot when it's not veggie time, first I'll try adding extra hay (even though the rack is not empty). This often "distracts" them and they eat the hay and forget about whining for veggies. If not, then I'll bring up a leaf or two of lettuce or endive. I don't like to give them too much extra veggies, but if they're really wheeking, I usually do give in.
But... I don't have the cage in sight/sound range of the fridge either. If they were doing this every time someone else in the family ate something from the fridge, I'd think you might end up giving too many veggies, and that could potentially cause loose stool or diarrhea.
What I might try in your situation is giving the morning veggies, and then making up a baggie of evening veggies. When they wheek for something during the day, take it from their evening baggie. That way you are serving the same amount of veggies as recommended, but allowing them to have them on their schedule. As long as they have hay and fresh water (and pellets, if they haven't eaten their daily allotment of those yet) they are not "starving", just being "piggies" and asking for extra.
